Particulate metal oxide

ABSTRACT

A particulate metal oxide having the mean length of the primary particles in the range from 50 to 90 nm, the mean width of the primary particles in the range from 5 to 20 nm, and the median volume particle diameter of the secondary particles is less than 45 nm. The metal oxide can be used in a sunscreen product that exhibits both effective UV protection and improved transparency.

FIELD OF INVENTION

The present invention relates to a particulate metal oxide, a metaloxide dispersion and in particular to the use thereof in a sunscreenproduct.

BACKGROUND OF THE INVENTION

Metal oxides such as titanium dioxide, zinc oxide and iron oxides havebeen employed as attenuators of ultraviolet light in applications suchas sunscreens, plastics films and resins. Due to the increased awarenessof the link between ultraviolet light and skin cancer, there has been anincreasing requirement for ultraviolet light protection in everydayskincare and cosmetics products. Unfortunately, existing commerciallyavailable metal oxide products, such as titanium dioxide, are notsufficiently transparent and can have an unacceptable whitening effectwhen used on the skin. There is a need for a metal oxide which exhibitsimproved transparency, reduced whitening, and provides broad spectrumultraviolet light protection.

REVIEW OF THE PRIOR ART

GB-2206339-A is directed to an oil dispersion of titanium dioxideparticles having a particle size in the range from 0.01 to 0.15 microns.GB-2205088-A discloses particulate acicular titanium dioxide having acoating layer of aluminium oxide and silicon oxide.

GB-2226018-A is directed to an aqueous dispersion of particulateacicular titanium dioxide containing an acrylic dispersing agent.

SUMMARY OF THE INVENTION

We have now surprisingly discovered an improved metal oxide, whichovercomes or significantly reduces at least one of the aforementionedproblems.

Accordingly, the present invention provides a particulate metal oxidewherein the mean length of the primary particles is in the range from 50to 90 nm, the mean width of the primary particles is in the range from 5to 20 nm, and the median particle volume diameter of the secondaryparticles is less than 45 nm.

The present invention also provides a dispersion comprising particles ofmetal oxide in a dispersing medium wherein the mean length of theprimary particles is in the range from 50 to 90 nm, the mean width ofthe primary particles is in the range from 5 to 20 nm, and the medianparticle volume diameter of the secondary particles is less than 45 nm.

The invention further provides a particulate metal oxide wherein themean length of the primary particles is in the range from 55 to 85 nm,the mean width of the primary particles is in the range from 8 to 19 nm,and at least 70% of the primary particles have a length in the rangefrom 55 to 85 nm.

The invention further provides a particulate metal oxide, optionallyhydrophobic, having an extinction coefficient at 524 nm (E₅₂₄) in therange from 0.2 to 0.7 l/g/cm, an extinction coefficient at 450 nm (E₄₅₀)in the range from 0.5 to 1.5 l/g/cm, an extinction coefficient at 360 nm(E₃₆₀) in the range from 4 to 8 l/g/cm, an extinction coefficient at 308nm (E₃₀₈) in the range from 40 to 60 l/g/cm, a maximum extinctioncoefficient E(max) in the range from 50 to 70 l/g/cm, and a λ(max) inthe range from 270 to 290 nm.

The invention further provides a sunscreen product comprising a metaloxide or dispersion as defined herein.

The invention still further provides the use of a metal oxide ordispersion as defined herein in the manufacture of a sunscreen havingreduced whiteness.

Preferably the metal oxide used in the present invention comprises anoxide of titanium, zinc or iron, and most preferably the metal oxide istitanium dioxide.

The preferred titanium dioxide particles comprise anatase and/or rutilecrystal form. The titanium dioxide particles preferably comprise a majorportion of rutile, more preferably greater than 60% by weight,particularly greater than 70%, and especially greater than 80% by weightof rutile. The titanium dioxide particles preferably comprise in therange from 0.01 to 5%, more preferably 0.1 to 2%, and particularly 0.2to 0.5% by weight of anatase. In addition, the titanium dioxideparticles preferably comprise less than 40%, more preferably less than30%, and particularly less than 25% by weight of amorphous titaniumdioxide. The basic particles may be prepared by standard procedures,such as using the chloride process, or by the sulphate process, or byhydrolysis of an appropriate titanium compound such as titaniumoxydichloride or an organic or inorganic titanate, or by oxidation of anoxidisable titanium compound, e.g. in the vapour state. The titaniumdioxide particles are preferably prepared by the hydrolysis of atitanium compound, particularly of titanium oxydichloride.

The individual or primary metal oxide particles are preferably acicularin shape and have a long axis (maximum dimension or length) and shortaxis (minimum dimension or width). The third axis of the particles (ordepth) is preferably approximately the same dimensions as the width. Thesize of the primary particles can be suitably measured using electronmicroscopy. The size of a particle can be determined by measuring thelength and width of a filler particle selected from a photographic imageobtained by using a transmission electron microscope. Mean values can bedetermined from the measurements of at least 300 particles, as describedherein.

The mean length by number of the primary metal oxide particles is in therange from 50 to 90 nm, preferably 55 to 85 nm, more preferably 60 to 80nm, particularly 65 to 77 nm, and especially 69 to 73 nm. The mean widthby number of the particles is in the range from 5 to 20 nm, preferably 8to 19 nm, more preferably 10 to 18 nm, particularly 12 to 17 nm, andespecially 14 to 16 nm.

The size distribution of the primary metal oxide particles can also havea significant effect on the final properties of, for example, asunscreen product comprising the metal oxide. In a preferred embodimentof the invention suitably at least 40%, preferably at least 50%, morepreferably at least 60%, particularly at least 70%, and especially atleast 80% by number of particles have a length within the abovepreferred ranges given for the mean length. In addition, suitably atleast 40%, preferably at least 50%, more preferably at least 60%,particularly at least 70%, and especially at least 80% by number ofparticles have a width within the above preferred ranges given for themean width.

The primary metal oxide particles preferably have a mean aspect ratiod₁:d₂ (where d₁ and d₂, respectively, are the length and width of theparticle) in the range from 2.0 to 8.0:1, more preferably 3.0 to 6.5:1,particularly 4.0 to 6.0:1, and especially 4.5 to 5.5:1.

The primary metal oxide particles preferably have a median volumeparticle diameter (equivalent spherical diameter corresponding to 50% ofthe volume of all the particles, read on the cumulative distributioncurve relating volume % to the diameter of the particles—often referredto as the “D(v,0.5)” value), measured as herein described, in the rangefrom 25 to 35 nm, more preferably 27 to 33 nm, particularly 28 to 32 nm,and especially 29 to 31 nm.

In one embodiment of the invention, the primary metal oxide particlesaggregate to form clusters or agglomerates of secondary particlescomprising a plurality of metal oxide primary particles. The aggregationprocess of the primary metal oxide particles may take place during theactual synthesis of the metal oxide and/or during subsequent processing.The mean number of primary metal oxide particles present in thesecondary particles according to the present invention is suitably inthe range from 1 to 10, preferably 1.05 to 8, more preferably 1.1 to 5,particularly 1.3 to 3, and especially 1.4 to 2.0. Thus, statistically atleast some of the secondary particles may contain only one primaryparticle, i.e. some primary particles are also secondary particles. Theterm “secondary” particles is partly used as a label to relate t6particle size results obtained using a particular technique, asdescribed herein.

The particulate metal oxide according to the present invention has amedian volume particle diameter (equivalent spherical diametercorresponding to 50% of the volume of all the particles, read on thecumulative distribution curve relating volume % to the diameter of theparticles—often referred to as the “D(v,0.5)” value)) of the secondaryparticles, measured as herein described, of less than 45 nm, preferablyin the range from 30 to 40 nm, more preferably 32 to 38 nm, particularly33 to 37 nm, and especially 34 to 36 nm.

The size distribution of the secondary metal oxide particles can also bean important parameter in obtaining, for example, a sunscreen producthaving the required properties. The metal oxide particles preferablyhave no more than 16% by volume of particles having a volume diameter ofless than 20 nm, more preferably less than 24 nm, particularly less than28 nm, and especially less than 32 nm. In addition, the metal oxideparticles preferably have more than 84% by volume of particles having avolume diameter of less than 70 nm, more preferably less than 60 nm,particularly less than 50 nm, and especially less than 40 nm.

It is preferred that none of the secondary metal oxide particles shouldhave an actual particle size exceeding 150 run. Particles exceeding sucha size may be removed by milling processes which are known in the art.However, milling operations are not always totally successful ineliminating all particles greater than a chosen size. In practice,therefore, the size of 95%, preferably 99% by volume of the particlesshould not exceed 150 rnm.

Particle size of the secondary metal oxide particles described hereinmay be measured by electron microscope, coulter counter, sedimentationanalysis and static or dynamic light scattering. Techniques based onsedimentation analysis are preferred. The median particle size may bedetermined by plotting a cumulative distribution curve representing thepercentage of particle volume below chosen particle sizes and measuringthe 50th percentile. The median particle volume diameter of thesecondary metal oxide particles is suitably measured using a Brookhavenparticle sizer, as described herein.

In a particularly preferred embodiment of the invention, the metal oxideparticles have a BET specific surface area, measured as describedherein, of greater than 40, more preferably in the range from 50 to 100,particularly 60 to 90, and especially 65 to 75 m²/g.

The particles of metal oxide may comprise substantially pure metaloxide, but in one embodiment of the invention the particles have aninorganic coating. For example, metal oxide particles, such as titaniumdioxide, may be coated with oxides of other elements such as oxides ofaluminium, zirconium or silicon, or mixtures thereof such as alumina andsilica as disclosed in GB-2205088-A, the teaching of which isincorporated herein by reference. The preferred amount of inorganiccoating is in the range from 2% to 25%, more preferably 4% to 20%,particularly 6% to 15%, and especially 8% to 12% by weight, calculatedwith respect to the weight of metal oxide core particles. The inorganiccoating may be applied using techniques known in the art. A typicalprocess comprises forming an aqueous dispersion of metal oxide particlesin the presence of a soluble salt of the inorganic element whose oxidewill form the coating. This dispersion is usually acidic or basic,depending upon the nature of the salt chosen, and precipitation of theinorganic oxide is achieved by adjusting the pH of the dispersion by theaddition of acid or alkali, as appropriate.

In a particularly preferred embodiment of the invention, the particlesof metal oxide are coated in order to render them hydrophobic. Suitablecoating materials are water-repellent, preferably organic, and includefatty acids, preferably fatty acids containing 10 to 20 carbon atoms,such as lauric acid, stearic acid and isostearic acid, salts of theabove fatty acids such as sodium salts and aluminium salts, fattyalcohols, such as stearyl alcohol, and silicones such aspolydimethylsiloxane and substituted polydimethylsiloxanes, and reactivesilicones such as methylhydrosiloxane and polymers and copolymersthereof. Stearic acid and/or salt thereof is particularly preferred. Theorganic coating may be applied using any conventional process.Typically, metal oxide particles are dispersed in water and heated to atemperature in the range 50° C. to 80° C. A fatty acid, for example, isthen deposited on the metal oxide particles by adding a salt of thefatty acid (e.g. sodium stearate) to the dispersion, followed by anacid. Alternatively, the metal oxide core particles can be mixed with asolution of the water-repellent material in an organic solvent, followedby evaporation of the solvent. In an alternative embodiment of theinvention, the water-repellant material can be added directly to thedispersion, during preparation thereof, such that the hydrophobiccoating is formed in situ. Generally, the particles are treated with upto 25%, more preferably in the range from 3% to 20%, particularly 6% to17%, and especially 10% to 15% by weight of organic material, preferablyfatty acid, calculated with respect to the metal oxide core particles.

In a preferred embodiment of the invention, the metal oxide particlesmay be coated with both an inorganic and an organic coating, eithersequentially or as a mixture. It is preferred that the inorganiccoating, preferably alumina, is applied first followed by the organiccoating, preferably fatty acid and/or salt thereof. Thus, preferredmetal oxide particles according to the present invention comprise (i) inthe range from 60% to 98%, more preferably 65% to 95%, particularly 70%to 80%, and especially 72% to 78% by weight of metal oxide, preferablytitanium dioxide, with respect to the total weight of the particles,(ii) in the range from 0.5% to 15%, more preferably 2% to 12%,particularly 5% to 10%, and especially 6% to 9% by weight of inorganiccoating, preferably alumina, with respect to the total weight of theparticles, and (iii) in the range from 1% to 21%, more preferably 4% to18%, particularly 7% to 15%, and especially 9% to 12% by weight oforganic coating, preferably fatty acid and/or salt thereof, with respectto the total weight of the particles. Such metal oxide particles providea surprising combination of both improved photostability anddispersibility, particularly when dispersed in a suitable organicmedium.

The metal oxide particles used in the present invention exhibit improvedtransparency preferably having an extinction coefficient at 524 nm(E₅₂₄), measured as herein described, of less than 2.0, more preferablyin the range from 0.1 to 1.0, particularly 0.2 to 0.7, and especially0.3 to 0.5 l/g/cm. In addition, the metal oxide particles preferablyhave an extinction coefficient at 450 nm (E₄₅₀), measured as hereindescribed, of less than 3.0, more preferably in the range from 0.1 to2.0, particularly 0.5 to 1.5, and especially 0.7 to 1.0 l/g/cm. Themetal oxide particles exhibit effective UV absorption, suitably havingan extinction coefficient at 360 nm (E₃₆₀), measured as hereindescribed, of greater than 3, preferably in the range from 4 to 10, morepreferably 5 to 8, particularly 5.5 to 7.5, and especially 6 to 7l/g/cm.

The metal oxide particles also preferably having an extinctioncoefficient at 308 nm (E₃₀₈), measured as herein described, of greaterthan 30, more preferably in the range from 35 to 65, particularly 40 to60, and especially 45 to 55 l/g/cm.

The metal oxide particles preferably have a maximum extinctioncoefficient E(max), measured as herein described, in the range from 40to 80, more preferably from 45 to 75, particularly 50 to 70, andespecially 55 to 65 l/g/cm. The metal oxide particles preferably have aλ(max), measured as herein described, in the range from 260 to 290, morepreferably 265 to 285, particularly 268 to 280, and especially 270 to275 nm.

The metal oxide particles suitably exhibit reduced whiteness, preferablyhaving a change in whiteness ΔL of a sunscreen product containing theparticles, measured as herein described, of less than 3, more preferablyin the range from 0.5 to 2.5, and particularly 1.0 to 2.0. In addition,a sunscreen product containing the particles preferably has a whitenessindex, measured as herein described, of less than 100%, more preferablyin the range from 10% to 80%, particularly 20% to 60%, and especially30% to 50%.

The metal oxide particles suitably have reduced photogreying, preferablyhaving a photogreying index, measured as herein described, of less than15, more preferably in the range from 1 to 10, particularly 2 to 7, andespecially 3 to 5. The particulate metal oxide according to the presentinvention may be in the form of a free-flowing powder. A powder havingthe required particle size for the secondary metal oxide particles, asdescribed herein, may be produced by milling processes known in the art.The final milling stage of the metal oxide is suitably carried out indry, gas-borne conditions to reduce aggregation. A fluid energy mill canbe used in which the aggregated metal oxide powder is continuouslyinjected into highly turbulent conditions in a confined chamber wheremultiple, high energy collisions occur with the walls of the chamberand/or between the aggregates. The milled powder is then carried into acyclone and/or bag filter for recovery. The fluid used in the energymill may be any gas, cold or heated, or superheated dry steam.

The particulate metal oxide may be formed into a slurry, or preferably aliquid dispersion, in any suitable aqueous or organic liquid medium. Byliquid dispersion is meant a true dispersion, i.e., where the solidparticles are stable to aggregation. The particles in the dispersion arerelatively uniformly dispersed and resistant to settling out onstanding, but if some settling out does occur, the particles can beeasily redispersed by simple agitation. Cosmetically acceptablematerials are preferred as the liquid medium. A useful organic medium isa liquid oil such as vegetable oils, e.g. fatty acid glycerides, fattyacid esters and fatty alcohols. A preferred organic medium is a siloxanefluid, especially a cyclic oligomeric dialkylsiloxane, such as thecyclic pentamer of dimethylsiloxane known as cyclomethicone. Alternativefluids include dimethylsiloxane linear oligomers or polymers having asuitable fluidity and phenyltris(trimethylsiloxy)silane (also known asphenyltrimethicone).

Examples of suitable organic media include avocado oil, C12-15 alkylbenzoate, C12-15 alkyl ethylhexanoate, C12-15 alkyl lactate, C12-15alkyl salicylate, C13-14 isoparaffin, C18-36 acid glycol ester, C18-36acid triglyceride, caprylic/capric glycerides, caprylic/caprictriglyceride, caprylic/capric/lauric triglyceride,caprylic/capric/linoleic triglyceride, caprylic/capric/myristic/stearictriglyceride, caprylic/capric/stearic triglyceride, castor oil, castoroil-silicone ester, cetearyl ethylhexanoate, cetearyl isononanoate,cetearyl palmitate, cetearyl stearate, cetyl dimethicone, cetyldimethicone copolyol, cetyl ethylhexanoate, cetyl glycol isostearate,cetyl isononanoate, cetyl lactate, cetyl myristate, cetyl oleate, cetylpalmitate, cetyl ricinoleate, cetyl stearate, cocoglycerides, coconutoil, cyclomethicone, cyclopentasiloxane, cyclotetrasiloxane, decylisostearate, decyl oleate, decyl polyglucoside, dibutyl adipate,diethylhexyl dimer dilinoleate, diethylhexyl malate, diisopropyladipate, diisopropyl dimer dilinoleate, diisostearoyl trimethylolpropanesiloxy silicate, diisostearyl adipate, diisostearyl dimer dilinoleate,diisostearyl malate, diisostearyl trimethylolpropane siloxy silicate,dilauroyl trimethylolpropane siloxy silicate, dilauryltrimethylolpropane siloxy silicate, dimethicone, dimethicone copolyol,dimethicone propyl PG-betaine, dimethiconol, dimethyl isosorbide,dioctyl maleate, dioctylodedecyl dimer dilonoleate, ethylhexyl benzoate,ethylhexyl cocoate, ethylhexyl dimethyl PABA, ethylhexyl ethylhexanoate,ethylhexyl hydroxystearate, ethylhexyl hydroxystearate benzoate,ethylhexyl isononanoate, ethylhexyl isopalmitate, ethylhexylisostearate, ethylhexyl laurate, ethylhexyl methoxycinnamate, ethylhexylmyristate, ethylhexyl neopentanoate, ethylhexyl oleate, ethylhexylpalmitate, ethylhexyl salicylate, ethylhexyl stearate, glyceryl caprate,glyceryl caprylate, glyceryl caprylate/caprate, glyceryl cocoate,glyceryl dilaurate, glyceryl dioleate, glyceryl hydroxystearate,glyceryl isostearate, glyceryl laurate, glyceryl oleate, glycol oleate,glycol ricinoleate, helianthus annuus (hybrid sunflower) seed oil,helianthus annuus (sunflower) seed oil, homosalate, isoamyl laurate,isoamyl p-methoxycinnamate, isocetyl alcohol, isocetyl behenate,isocetyl ethylhexanoate, isocetyl isostearate, isocetyl laurate,isocetyl linoleoyl stearate, isocetyl myristate, isocetyl palmitate,isocetyl salicylate, isocetyl stearate, isocetyl stearoyl stearate,isohexadecane, isononyl isononanoate, isopropyl C12-15-pareth-9carboxylate, isopropyl isostearate, isopropyl lanolate, isopropyllaurate, isopropyl linoleate, isopropyl methoxycinnamate, isopropylmyristate, isopropyl oleate, isopropyl palmitate, isopropylPPG-2-isodeceth-7 carboxylate, isopropyl ricinoleate, isopropylstearate, isostearic acid, isostearyl alcohol, isostearylethylhexanoate, isostearyl isononanoate, isostearyl isostearate,isostearyl lactate, isostearyl myristate, isostearyl neopentanoate,isostearyl palmitate, isostearyl stearoyl stearate, jojoba oil, lanolin(lanolin oil), maleated soybean oil, myristyl isostearate, myristyllactate, myristyl myristate, myristyl neopentanoate, myristyl stearate,octocrylene, octyldecanol, octyldodecanol, oenothera biennis (eveningprimrose oil), paraffinum liquidum (mineral oil), PCA dimethicone,pentaerythrityl tetraisononanoate, pentaerythrityl tetraisostearate,perfluoropolymethylisopropyl ether, persea gratissima (avocado oil),phenyl trimethicone, PPG-15 stearyl ether, propylene glycol ceteth-3acetate, propylene glycol dicaprylate, propylene glycoldicaprylate/dicaprate, propylene glycol dipelargonate, propylene glycoldistearate, propylene glycol isoceteth-3 acetate, propylene glycolisostearate, propylene glycol laurate, proylene glycol ricinoleate,propylene glycol stearate, prunus dulcis (sweet almond oil), squalane,squalene, tricaprylin, tricaprylyl citrate, tridecyl ethylhexanoate,tridecyl neopentanoate, tridecyl stearoyl stearate, triethylhexanoin,triethylhexyl citrate, trihydroxystearin, triisocetyl citrate,triisostearin, triisostearyl citrate, trimethylolpropane triisostearate,trimethylsiloxysilicate, triticum vulgare (wheat germ oil), vitisvinifera (grape) seed oil, and mixtures thereof.

The metal oxide dispersions may also contain a dispersing agent in orderto improve the properties thereof. The dispersing agent is preferablypresent in the range from 1% to 50%, more preferably 3% to 30%,particularly 5% to 20%, and especially 8% to 15% by weight based on thetotal weight of metal oxide particles.

Suitable dispersing agents for use in an organic medium includesubstituted carboxylic acids, soap bases and polyhydroxy acids.Typically the dispersing agent can be one having a formula X.CO.AR inwhich A is a divalent bridging group, R is a primary secondary ortertiary amino group or a salt thereof with an acid or a quaternaryammonium salt group and X is the residue of a polyester chain whichtogether with the —CO— group is derived from a hydroxy carboxylic acidof the formula HO—R′—COOH. As examples of typical dispersing agents arethose based on ricinoleic acid, hydroxystearic acid, hydrogenated castoroil fatty acid which contains in addition to 12-hydroxystearic acidsmall amounts of stearic acid and palmitic acid. Dispersing agents basedon one or more polyesters or salts of a hydroxycarboxylic acid and acarboxylic acid free of hydroxy groups can also be used. Compounds ofvarious molecular weights can be used. Other suitable dispersing agentsare those monoesters of fatty acid alkanolamides and carboxylic acidsand their salts. Alkanolamides are based on ethanolamine, propanolamineor aminoethyl ethanolamine for example. Alternative dispersing agentsare those based on polymers or copolymers of acrylic or methacrylicacids, e.g. block copolymers of such monomers. Other dispersing agentsof similar general form are those having epoxy groups in the constituentradicals such as those based on the ethoxylated phosphate esters. Thedispersing agent can be one of those commercially referred to as a hyperdispersant. Suitable dispersing agents for use in an aqueous mediuminclude a polymeric acrylic acid or a salt thereof. Partially or fullyneutralized salts are usable e.g. the alkali metal salts and ammoniumsalts. Examples of dispersing agents are polyacrylic acids, substitutedacrylic acid polymers, acrylic copolymers, sodium and/or ammonium saltsof polyacrylic acids and sodium and/or ammonium salts of acryliccopolymers. Such dispersing agents are typified by polyacrylic aciditself and sodium or ammonium salts thereof as well as copolymers of anacrylic acid with other suitable monomers such as a sulphonic acidderivative such as 2-acrylamido 2-methyl propane sulphonic acid.Comonomers polymerisable with the acrylic or a substituted acrylic acidcan also be one containing a carboxyl grouping. Usually the dispersingagents have a molecular weight of from 1,000 to 10,000 and aresubstantially linear molecules.

A surprising feature of the present invention is that dispersions,particularly in an organic medium, can be produced which contain atleast 35%, preferably at least 40%, more preferably at least 45%,particularly at least 50%, especially at least 55%, and generally up to60% by weight of the total weight of the dispersion, of metal oxideparticles.

Alternatively, the particulate metal oxide may be in the form of alotion or cream of a solid and/or semi-solid dispersion. Suitable solidor semi-solid dispersions may contain, for example, in the range from50% to 90%, preferably 60% to 85% by weight of particulate metal oxideaccording to the present invention, together with any one or more of theliquid media disclosed herein, or a high molecular polymeric material,such as a wax.

The particulate metal oxide and dispersions of the present invention areuseful as ingredients for preparing sunscreen compositions, especiallyin the form of emulsions. The dispersion may further containconventional additives suitable for use in the intended application,such as conventional cosmetic ingredients used in sunscreens. Theparticulate metal oxide according to the present invention may providethe only ultraviolet light attenuators in a sunscreen product accordingto the invention, but other sunscreen agents, such as other metal oxidesand/or other organic materials may also be added. For example, thepreferred titanium dioxide particles described herein may be used incombination with existing commercially available titanium dioxide and/orzinc oxide sunscreens. Suitable organic sunscreens for use with metaloxide according to the invention include p-methoxy cinnamic acid esters,salicylic acid esters, p-amino benzoic acid esters, non-sulphonatedbenzophenone derivatives, derivatives of dibenzoyl methane and esters of2-cyanoacrylic acid. Specific examples of useful organic sunscreensinclude benzophenone-1, benzophenone-2, benzophenone-3, benzophenone-6,benzophenone-8, benzophenone-12, isopropyl dibenzoyl methane, butylmethoxy dibenzoyl methane, ethyl dihydroxypropyl PABA, glyceryl PABA,octyl dimethyl PABA, octyl methoxycinnamate, homosalate, octylsalicylate, octyl triazone, octocrylene, etocrylene, menthylanthranilate, and 4-methylbenzylidene camphor.

The invention is illustrated by the following non-limiting examples. Inthis specification, the following test methods have been used todetermine certain properties of the metal oxide particles:

1) Particle Size Measurement of Primary Metal Oxide Particles

A small amount of metal oxide, typically 2 mg, was pressed intoapproximately 2 drops of an oil, for one or two minutes using the tip ofa steel spatula. The resultant suspension was diluted with solvent and acarbon-coated grid suitable for transmission electron microscopy waswetted with the suspension and dried on a hot-plate. Approximately 18cm×21 cm photographs were produced at an appropriate, accuratemagnification. Generally about 300-500 crystals were displayed at about2 diameters spacing. A minimum number of 300 primary particles weresized using a transparent size grid consisting of a row of circles ofgradually increasing diameter, representing spherical crystals. Undereach circle a series of ellipsoid outlines were drawn representingspheroids of equal volume and gradually increasing eccentricity. Thebasic method assumes log normal distribution standard deviations in the1.2-1.6 range (wider crystal size distributions would require many morecrystals to be counted, for example of the order of 1000). Thedispersion method described above has been found to be suitable forproducing almost totally dispersed distributions of primary metal oxideparticles whilst introducing minimal crystal fracture. Any residualaggregates (or secondary particles) are sufficiently well defined thatthey, and any small debris, can be ignored, and effectively only primaryparticles included in the count.

Mean length, mean width and length/width size distributions of theprimary metal oxide particles can be calculated from the abovemeasurements. Similarly, the median particle volume diameter of theprimary particles can also be calculated.

2) Median Particle Volume Diameter Measurement of Secondary Metal OxideParticles

A dispersion of metal oxide particles was produced by mixing 10 g ofpolyhydroxystearic acid with 90 g of isopropyl myristate, and thenadding 100 g of metal oxide into the solution. The mixture was passedthrough a horizontal bead mill, operating at approximately 1500 r.p.m.and containing zirconia beads as grinding media for 15 minutes. Thedispersion of metal oxide particles was diluted to between 30 and 40 g/lby mixing with isopropyl myristate. The diluted sample was analyzed onthe Brookhaven BI-XDC particle sizer in centrifugation mode, and themedian particle volume diameter measured.

3) BET Specific Surface Area of Metal Oxide Particles

The single point BET specific surface area was measured using aMicromeritics Flowsorb II 2300.

4) Change in Whiteness and Whiteness Index

A sunscreen formulation was coated on to the surface of a glossy blackcard and drawn down using a No 2 K bar to form a film of 12 μm wetthickness. The film was allowed to dry at room temperature for 10minutes and the whiteness of the coating on the black surface (L_(F))measured using a Minolta CR300 colourimeter. The change in whiteness ΔLwas calculated by subtracting the whiteness of the substrate (L_(S))from the whiteness of the coating (L_(F)). The whiteness index is thepercentage change in whiteness ΔL compared to a standard titaniumdioxide (=100% value) (Tayca MT100T (ex Tayca Corporation)).

5) Photogreying Index

A metal oxide dispersion was placed inside a 6 cm×3 cm acrylic cell(containing a 2 cm×1.5 cm space), and the cell made air tight byclamping a glass slide over the top, ensuring that no air bubbles werepresent. The initial whiteness (L_(I)) was measured using a MinoltaCR300 colourimeter. The cell was then placed on a turntable revolving at30 rpm and exposed to UV light for 2 hours (a UV lamp containing 4TL29D,16/09 tubes mounted 12 cm from the cell), and the whiteness(L_(T)) remeasured. The photogreying index ΔL=L_(I)−L_(T).

EXAMPLES Example 1

2 moles of titanium oxydichloride in acidic solution were reacted with 6moles of NaOH in aqueous solution, with stirring, in a 3 litre glassvessel. After the initial reaction phase, the temperature was increasedto above 70° C., by heating at a rate of approximately 1° C./min, andtirring continued for at least another 60 minutes. The mixture was thenneutralised by the addition of NaOH in aqueous solution, and allowed tocool below 70° C.

To the resultant dispersion, an alkaline solution of sodium aluminatewas added, equivalent to 7% by weight Al₂O₃ on TiO₂ weight. Thetemperature was maintained below 70° C. during the addition. Thetemperature was then increased to above 70° C., and stirred for at leastanother 10 minutes. Sodium stearate equivalent to 12.5% by weightstearate on weight of TiO₂ was added, and the reaction mixture againstirred for at least a further 10 minutes.

The dispersion was neutralised to pH 6.5 to 7.0 by adding 36%hydrochloric acid solution over 30 minutes. The neutralised slurry wasaged for 15 minutes whilst being stirred. The slurry was then filteredto produce a filter cake which was then washed repeatedly withdemineralised water until the cake conductivity (when a small sample wasreslurried to 100 g/l) was less than 500 □s. The filter cake was driedin an oven at 105° C. for 16 hours and then micropulverised using ahammer mill to produce particulate titanium dioxide.

A dispersion was produced by mixing 10 g of polyhydroxystearic acid with90 g of isopropyl myristate, and then adding 100 g of the titaniumdioxide produced above into the solution. The mixture was passed througha horizontal bead mill, operating at approximately 1500 r.p.m. andcontaining zirconia beads as grinding media for 15 minutes.

The dispersion was subjected to the test procedures described herein,and the titanium dioxide exhibited the following properties:

Primary Particles

-   i) Mean length=71 nm,-   ii) Mean width=15.2 nm,-   iii) Mean aspect ratio=4.7,-   iv) Number of particles having a length within 55 to 85 nm=79% and-   v) D(v,0.5)=30nm.    Secondary Particles-   i) D (v,0.5)=35nm,-   ii) 16% by volume of particles have volume diameter less than 27nm,-   iii) 84% by volume of particles have volume diameter less than 46    nm,-   iv) BET specific surface area=70 m2/g, and-   v) Photogreying index=7.

A sample (0.1 g) of the milled titanium dioxide dispersion producedabove was diluted with cyclohexane (100 ml). This diluted sample wasthen further diluted with cyclohexane in the ratio sample:cyclohexane of1: 19. The total dilution was 1:20,000. The diluted sample was thenplaced in a spectrophotometer (Perkin-Elmer Lambda 2 UV/VISSpectrophotometer) with a 1 cm path length and the absorbance, of UV andvisible light measured. Extinction coefficients were calculated from theequation A=E.c.l, where A=absorbance, E=extinction coefficient in litresper gram per cm, c=concentration in grams per litre, and l=path lengthin cm.

The results were as follows: E₅₂₄ E₄₅₀ E₃₀₈ E₃₆₀ E(max) λ(max) 0.4 0.943.4 5.6 64.7 273

Example 2

This titanium dioxide dispersion produced in Example 1 was used toprepare a sunscreen formulation having the following composition. % byweight Phase A: Arlacel P135 (ex Uniqema) 2.0 Arlamol HD P135 (exUniqema) 5.0 AEC Cyclomethicone (Pentamer) (ex A&E Connock Ltd) 5.6Jojoba Oil) (ex A&E Connock Ltd) 4.0 Arlamol E (ex Uniqema) 2.4Candelilla Wax (ex Eggar&Co Chemicals Ltd) 1.0 Magnesium Stearate 0.7Titanium Dioxide dispersion produced above 12.0 Phase B: Allantoin (exUniqema) 0.2 Atlas G-2330 (cx Uniqema) 3.0 D-Panthenol (EX RocheProducts Ltd) 0.8 Magnesium Sulfate 0.7 Aqua (Water) 61.6 Preservative1.0

The ingredients of phase A were mixed and heated to 75-80 ° C. Theingredients of phase B were mixed and heated to 75-80 ° C. and slowlyadded to phase A with intensive mixing, followed by stirring with aSilverson mixer for 2 minutes. Finally, the mixture was cooled withintensive stirring.

The change in whiteness ΔL was 1.84, and the whiteness index was 60%,for the above sunscreen product. The Sun Protection Factor of thesunscreen product was determined using the in vitro method of Diffey andRobson, J. Soc. Cosmet. Chem. Vol. 40, pp 127-133, 1989, and a value of10.7 was obtained.

Example 3

The procedure of Example 1 was repeated except that the micropulverisedparticulate titanium dioxide was mixed at a concentration of 100 g/lwith 9:1 isopropyl myristate/polyhydroxystearic acid, and milled with150 μm glass beads (Ballotini Grade II) in a small scale sand mill. Theresultant titanium dioxide dispersion had the following extinctioncoefficient values: E₅₂₄ E₄₅₀ E₃₀₈ E₃₆₀ E(max) λ(max) 0.2 0.6 41.8 4.762.1 274

The above examples illustrate the improved properties of a particulatemetal oxide, dispersion and sunscreen product according to the presentinvention.

1. A particulate metal oxide wherein the mean length of the primaryparticles is in the range from 50 to 90 nm, the mean width of theprimary particles is in the range from 5 to 20 nm, and the medianparticle volume diameter of the secondary particles is less than 45 nm.2. A dispersion comprising particles of metal oxide in a dispersingmedium wherein the mean length of the primary particles is in the rangefrom 50 to 90 nm, the mean width of the primary particles is in therange from 5 to 20 nm, and the median particle volume diameter of thesecondary particles is less than 45 nm.
 3. A metal oxide or dispersionaccording to claim 1 wherein the primary particles have a mean length inthe range from 55 to 85 nm and a mean width in the range from 8 to 19nm.
 4. A metal oxide or dispersion according to claims 1 wherein atleast 70% of the particles have a length in the range from 55 to 85 nm.5. A metal oxide or dispersion according to claim 4 wherein at least 70%of the particles have a length in the range from 60 to 80 nm.
 6. A metaloxide or dispersion according to claim 1 wherein the median particlevolume diameter of the primary particles is in the range from 28 to 32nm.
 7. A metal oxide or dispersion according to claim 1 wherein themedian particle volume diameter of the secondary particles is in therange from 32 to 38 nm.
 8. A metal oxide or dispersion according toclaim 1 wherein no more than 16% by volume of the secondary particleshave a volume diameter of less than 24 nm.
 9. A metal oxide ordispersion according to claim 1 wherein more than 84% by volume of thesecondary particles have a volume diameter of less than 60 nm.
 10. Ametal oxide or dispersion according to claim 9 wherein more than 84% byvolume of the secondary particles have a volume diameter of less than 50nm.
 11. A metal oxide or dispersion according to claim 1 wherein themean number of primary particles present in the secondary particles isin the range from 1.3 to
 3. 12. A metal oxide or dispersion according toclaim 1 wherein the particles are hydrophobic.
 13. A metal oxide ordispersion according to claim 12 wherein the particles comprise anorganic water repellant coating.
 14. A metal oxide or dispersionaccording to claim 1 wherein the particles comprise (i) in the rangefrom 65% to 95% by weight of titanium dioxide, (ii) in the range from 2%to 12% by weight of inorganic coating, preferably alumina, and (iii) inthe range from 4% to 18% by weight of organic coating, preferably fattyacid and/or salt thereof, all with respect to the total weight of theparticles.
 15. A dispersion according to claim 2 comprising an organicmedium and at least 45% by weight of metal oxide particles.
 16. Adispersion according to claim 2 comprising an aqueous medium and atleast 35% by weight of metal oxide particles.
 17. A particulate metaloxide wherein the mean length of the primary particles is in the rangefrom 55 to 85 nm, the mean width of the primary particles is in therange from 8 to 19 nm, and at least 70% of the primary particles have alength in the range from 55 to 85 nm. 18-19. (canceled)
 20. A sunscreenproduct comprising a metal oxide or dispersion as defined in claim 1.21. A sunscreen product according to claim 20 which is transparent whenapplied to the skin and has a change in whiteness ΔL in the range from0.5 to 2.5.
 22. A sunscreen product according to claim 20 having awhiteness index in the range from 10% to 80%.
 23. The use of a metaloxide or dispersion as defined in claim 1 in the manufacture of asunscreen having reduced whiteness.